Hello,
1/ Contact your hosting provider first, ask about assistance from their side and check the server error log.
2/ Try to disable all the third-party plugins (including cache plugins), that do not come bundled with XStore https://www.8theme.com/documentation/xstore/plugins/included-plugins/ , clear the cache of the browser, and check then. Maybe one of the additional plugins is a reason for the problem.
Let us know results
Regards